1. Field of the Invention
The present invention relates to thermoelectrically active materials and generators containing them and processes for preparing and testing of said thermoelectrically active materials and arrays obtained therefrom.
2. Description of the Related Art
Thermoelectric generators as such have been known for a long time. Electrical charges are transported through an external electric circuit by p-doped and n-doped semiconductors which are heated on one side and cooled on the other, electrical work is performed on a load in the electric circuit. The efficiency of the conversion of heat into electrical energy which is achieved is limited thermodynamically by the Carnot efficiency. Thus, at a temperature of 1000 K on the hot side and 400 K on the xe2x80x9ccoldxe2x80x9d side, the possible efficiency would be (1000xe2x88x92400) divided by 1000=60%. Unfortunately efficiencies of only 10% are achieved today.
A good overview of effects and materials is given, for example, by Cronin B. Vining, ITS Short Course on Thermoelectricity, Nov. 8, 1993 Yokohama, Japan.
At present, thermoelectric generators are employed in space probes to generate direct currents, for cathodic corrosion protection of pipelines, for supplying light buoys and radio buoys with energy, for radio and TV operation. The advantages of thermoelectric generators are their exceptional reliability, they function independently of atmospheric conditions such as humidity, there is no vulnerable transport of matter, but only a transport of charges; the operating material is burnt continuously, even catalytically without a free flame, releasing only minor amounts of CO, NOx and unburned operating material; any operating material may be used from hydrogen via natural gas, petrol, kerosene, diesel fuel to biologically produced fuels such as rapeseed oil methyl ester.
Thus, thermoelectric energy conversion adapts very flexibly to future needs such as hydrogen economy or energy generation from regenerative energies.
A particularly attractive application would be the use for conversion into electrical energy in electrically powered vehicles. There would be no need for altering the existing filling station network. However, such an application would require efficiencies of more than 30%.
It is therefore an object of the present invention to provide novel, thermoelectrically active materials which make it possible to achieve higher efficiencies than previously. Thermoelectric materials are characterized by the so called Z factor (figure of merit)   Z  =                    α        2            *      σ        K  
where xcex1 is the Seebeck coefficient, "sgr" is the electrical conductivity and K is the thermal conductivity.
Closer analysis shows that the efficiency xcex7 derives from   η  =                              T          high                -                  T          low                            T        high              *                  M        -        1                    M        +                              T            high                                T            low                              
where
M=[1+{fraction (z/2)}(Thighxe2x88x92Tlow)]xc2xd
(see also Mat. Sci. and Eng. B29 (1995) 228).
The aim is therefore to provide a material having a very high Z value and high realizable temperature difference. From a solid state physics point of view, many problems are to be overcome here:
A high xcex1 implies a high electron mobility in the material; i.e. electrons (or holes in the case of p-conducting materials) must not be bound strongly to the atomic core. Materials which have a high electrical conductivity often have a high thermal conductivity (Wiedemann-Franz law), which is why it is not possible to influence Z in a favorable way. Currently used materials such as Bi2Te3, PbTe or SiGe are already a compromise. For example, alloying reduces the electrical conductivity less than the thermal conductivity. Because of that, use is preferably made of alloys such as (Bi2T3)90(Sb2T3)5(Sb2Se3)5 or Bi12Sb23Te65 as described in U.S. Pat. No. 5,448,109.
For thermoelectric materials of high efficiency, it is preferred that further boundary conditions be satisfied. In particular, they must be temperature-stable to be able to operate at operating temperatures of from 1000 to 1500 K for years without substantial loss of efficiency. This implies high temperature-stable phases per se, a stable phase composition and a negligible diffusion of alloy constituents into the adjacent contact materials.
We have found that this object is achieved by a thermoelectric generator comprising a p-doped or n-doped semiconductor material, wherein said semiconductor material is at least one ternary material selected from one of the following substance classes and formed by combining at least 2 compounds of the substance class:
(1) Silicides
U3Si5, BaSi2, CeSi2, GdSi, NdSi2, CoSi, CoSi2, CrSi2, FeSi, FeSi2, MnSi, MoSi2, WSi2, VSi, TiSi2, ZrSi2, VSi2, NbSi2 and TaSi2 
(2) Borides
UB2, UB4, UB12, CeB6, AlB12, CoB, CrB2, CrB4, FeB, MnB, MnB2, MnB12, MoB, MoB4, SiB4, SiB6, SiB12, TiB2, VB2, YB4, ZrB2, CuB24, NiB12, BaB6, MgB2, MgB4 and MgB12, where the aluminum-containing borides may additionally contain one carbon atom per boron atom,
(3) Germanides
U5Ge3, BaGe, GdGe, Dy5Ge3, Fr5Ge3 and Ce3Ge5 
(4) Tellurides, sulfides and selenides
LaS, NdS, Pr2S3, DyS, USe, BaSe, GdSe, LaSe, Nd3Se4, Nd2Se3, PrSe, FrSe, UTe, GdTe, LaTe, NdTe, PrTe, SmTe, DyTe and ErTe
(5) Antimonides
USb, CeSb, GdSb, LaSb, NdSb, PrSb and DySb, AlSb, CeSb, CrSb, FeSb, Mg3Sb2, Ni5Sb2 and CeSb3 and NiSb3 
(6) Plumbides
CePb, Gd5Pb3, La5Pb3 and Dy5Pb4, where, in the substance classes (1) to (6), up to 10 atom % of the elements may be replaced by Na, K, Rb, Cs, Zn, Cd, Al, Ga, Zr, Mg, S, Cu, Ag, Au, Ti, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni or mixtures thereof, providing they are not already present in the combinations.
(7) Semiconductor oxides
UO2, Bi2O3, CuO, Cu2O, SnO, PbO, ZnO, In2O3, WO3, V2O5, Sb2O3, CoO, NiO, Ce2O4, FeO, Fe2O3, NbO2, CeO2 and BaO,
where up to 10 mol % of the oxides may be replaced by Na2O, K2O, CdO, SrO, A12O3, Ga2O3, Cr2O3 or mixtures thereof. The semiconductor material is preferably a binary or ternary alloy from one of the substance classes (1) to (6) or a binary oxide from the substance class (7), where no oxides or elements are replaced as stated. According to another procedure, thermoelectrically active materials may be prepared by combining and reacting from 30 to 50% by weight, preferably from 35 to 40% by weight, of one or more of the semiconductor-forming elements B, Si, Ge, Sb, Bi, S, Se and Te with from 50 to 70% by weight, preferably from 60 to 65% by weight, of one or more of the elements Mg, Al, Fe, Ni, Co, Zn, Cd, Ti, Zr, Y, Cu, V, Mo, W, Mn, Nb, Ta and U. As described hereinafter, these materials are combined in a suitable combinatorial manner followed by reaction of the elemental mixtures at elevated temperatures to give the actual thermoelectrically active materials by solid state reaction.
The doping element content in the alloy is up to 0.1 atom % or from 1018 to 1020 charge carriers per cubic centimeter. Higher charge carrier concentrations result in disadvantageous recombinations and thus in a reduced charge mobility. Doping is achieved by means of elements which give rise to an excess or deficiency of electrons in the crystal lattice, for example by means of iodide for n-type semiconductors and by means of alkaline earth elements for p-type semiconductors, provided the semiconductor is a 3/5 or 3/6 semiconductor.
Another possibility of doping is controlled introduction of holes or electrons into the materials by means of substoichiometric or superstoichiometric compositions, removing the need for an additional doping step.
Doping elements may also be introduced by means of aqueous solutions of metal salts which are subsequently dried in the mixture. The metal cations are then reduced, for example with hydrogen at elevated temperatures, or they remain in the material without reduction. Preferably, p-type doping or n-type doping is achieved by selecting the mixing ratios of the compounds or p-type doping is achieved by means of alkali metals and n-type doping is achieved by means of Sb, Bi, Se, Te, Br or I (see WO 92/13811).
It is advantageous to use heavy elements which are known to have a low thermal work function. In particular, these are known to be U, Bi, Se, Te, Ce and Ba.